1. Field of the Invention
The present invention relates to an infrared-sensitive image foaming material, an infrared-sensitive positive-working planographic printing plate precursor using the same, and a method for manufacturing a planographic printing plate.
2. Description of the Related Art
Conventionally, a variety of photosensitive compositions have been used as visible image forming materials or planographic printing plate materials. In particular, the recent development of lasers in the field of planographic printing has been outstanding and, particularly, regarding solid-state lasers or semiconductor lasers capable of emitting light in the range from near-infrared to infrared, high capacity and small sized lasers have become readily available. As an exposure light source for directly making a plate from digital data such as computer data, these lasers are very useful.
A positive planographic printing plate precursor for an infrared laser has an alkali-soluble binder resin, an infrared absorbing agent that absorbs light and generates heat, and the like, as essential components. In an unexposed region (i.e., an image portion), such an infrared absorbing agent or the like functions as a development inhibitor which substantially decreases the solubility of a binder resin in a developing liquid, owing to the interaction between the infrared absorbing agent and the binder resin. Meanwhile, in an exposed region (i.e., a non-image portion), the interaction between the infrared absorbing agent and the binder resin is suppressed by the heat generated in the exposed region, whereby the exposed region dissolves in an alkaline developing liquid and a planographic printing plate is formed.
However, in a planographic printing material which includes such an infrared ray-sensitive positive-working image forming material, a difference (i.e., discrimination; hereinafter, may be referred to as “solubility discrimination”) between the strength of an image forming layer in an unexposed region and the solubility in an aqueous alkaline solution in an exposed region is not sufficiently obtained, and the development conditions (i.e., the development latitude) suitable for forming an image are restricted. Therefore, there have been concerns that a residual film is generated when development is performed using an exhausted developing liquid with low activity, or that an undesired decrease in the strength of an image portion occurs when development is performed using a developing liquid with high activity.
In order to solve the problem of development latitude, use of a recording layer which enables easier development of a non-image portion, that is, a recording layer which is formed from a material having a relatively higher solubility in an aqueous alkaline solution, might be considered. However, since such a recording layer also becomes chemically weak in the image region, there has been a problem in that durability during normal printing is poor and, further, the chemical resistance is poor; for example, the recording medium is susceptible to damage caused by a development solution, an ink cleaning solvent used during printing, a plate cleaner, or the like.
For the purpose of solving this problem, a method is proposed in which a multilayered recording layer is provided, and a lower layer having a high alkali solubility is provided in the vicinity of the recording layer, so that generation of a residual film after the upper layer has been removed by exposure is suppressed as a result of the high alkali solubility of the lower layer, whereby the development latitude is improved (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 11-218914).
When a multilayered image recording layer is to be formed, the image recording layer is usually formed by sequentially applying a lower layer and an upper layer. Since the interface between an upper layer and a lower layer needs to be distinctly defined in order to benefit from the advantages of the multilayered structure, it is important to prevent the upper layer and the lower layer from being mixed during application of the upper layer.
An example of a method of preventing interface mixing caused by the compatibility between an upper layer and a lower layer generally includes a method in which the polarity of a lower layer is increased by introducing into the lower layer a binder polymer having a polar group such as a sulfonamide, thereby increasing the difference in polarity between the upper layer and the lower layer. However, there is a problem in that the development latitude becomes relatively narrower because the polar group included in a sulfonamide or the like that is capable of preventing the interlayer mixing between the lower layer and the upper layer, has a relatively high pKa. On the other hand, there is a problem in that an upper layer cannot be formed successfully when a binder polymer having a carboxylic acid group, which provides excellent development latitude, is used in a lower layer, because interlayer mixing between the lower layer and an upper layer is likely to occur due to polarity problems.
For the purpose of improving image formability, development latitude, solubility discrimination, and durability of an unexposed region while taking advantage of the characteristics of the multilayered structure, a technique is proposed in which a multifunctional amine compound is added to either of the layers of the multilayered structure, preferably to a lower layer, to allow interactions between plural functional groups and a binder polymer at multiple points, thereby improving the strength of an unexposed portion (see, for example, JP-A No. 2005-181734). However, even by using this method, the solubility of exposed portion cannot be sufficiently improved and, therefore, further improvement is desired.